A. Field of the Invention
The present invention relates to machinery used for lifting and lowering heavy loads. More particularly, the invention relates to an auxiliary hoist control apparatus which is suspendible from a hoisting machine such as a crane and useable to support heavy loads. The invention relates specifically to a precision load positioner which utilizes a linear hydraulic actuator to raise and lower heavy loads incrementally with respect to a hoist, a load cell weight sensor and digital display which provides accurate and immediate indications of static-load weight as well as any load weight deviation resulting from a load contacting an obstruction while being raised or lowered, and a pressure relief mechanism for preventing over-pressurizing the actuator cylinder due to operator error.
B. Description of Background Art
Most hoisting machines, or hoists, such as fixed and mobile cranes or derricks used to manipulate heavy loads, are inherently limited in the precision with which they can vertically position a load. Thus, most hoists constructed to have a capacity for raising and lowering loads which weigh thousands of pounds are incapable of positioning such loads to a precision of much less than a few inches. However, there are many situations which require vertical positioning of a load with much greater precision. For example, proper placement of a horizontal beam with respect to a vertical column often requires greater positioning accuracy than attainable with existing hoists. Similarly, the lowering of an object such as a satellite or the upper stage of a rocket with respect to a lower stage of the rocket for mating the two, or lifting an upper object to de-mate it from another object, typically requires that the upper object be lowered or raised to a precision of much less than one inch, sometimes as small as a thousandth of an inch.
A partial solution to the problem of precise vertical control of heavy objects manipulatable by a hoist was disclosed by Hoover et al., in U.S. Pat. No. 2,500,459, Work Supporting Attachment For Hoists. The attachment disclosed in Hoover et al., includes an elongated cylinder which has an upwardly protruding connector eye, supportable from a crane hook, the cylinder being filled with hydraulic fluid above and below a piston fitted at the lower end thereof with a piston rod that protrudes through a lower end wall or bulkhead of the cylinder, and has at the lower end thereof a hook for suspending a load to be positioned. The attachment includes a hydraulic fluid storage area coupled at an upper end thereof by a port to the upper part of the interior of the cylinder and at a lower end thereof through a valve port and fluid passageway to the lower interior space of the cylinder below the piston. A spring located within the cylinder and disposed between the lower end of the piston and the lower bulkhead biases the piston to an upwardly retracted position. To lower a load incrementally with respect to the crane hook, the valve is opened for an interval sufficient to allow fluid beneath the piston to be expelled through the fluid reservoir and into the upper part at the cylinder above the piston in response to a downward tension force exerted by the load on the piston rod. The device disclosed in Hoover et al., was limited in usefulness because it included no means for raising a load in small, precise increments.
In U.S. Pat. No. 3,025,702, Merrill et al., Auxiliary Hoist Control, an apparatus was disclosed which could be suspended from a hoist hook, support a heavy load, and raise as well as lower the load incrementally relative to the hoist. The disclosed apparatus includes a body in which is located a vertically elongated inner cylinder containing a piston and piston rod which protrudes through a lower cylinder head that forms a bottom end wall of the body, and which has depending downwardly therefrom a load support eye. The upper end of the cylinder is closed by an upper cylinder head which has protruding upwardly therefrom an upper connector eye for suspension from a hoist hook. The portion of the cylinder below the piston is filled with hydraulic fluid which communicates through a fluid passageway, valve port and a down-valve to the lower portion of a vertically elongated annular volume formed between an outer wall of the inner cylinder and an inner wall of an outer coaxial cylinder. A hermetically sealing separator ring longitudinally slidably positioned in the annular volume between the inner and outer cylinders separates the annular volume into a lower storage chamber and an upper storage chamber. The lower storage chamber contains hydraulic fluid. The upper storage chamber contains a compressible fluid such as nitrogen gas which is introduced through a gas input port to pressurize the upper annular space above the separator ring, to a predetermined pressure.
In the Merrill apparatus, the down-valve provides for a controlled escape of a portion of the hydraulic fluid which supports the piston within the cylinder. When the down-valve is opened, hydraulic fluid pressurized by the weight of the piston, piston rod and load attached to the lower end of the piston rod escapes through the valve into the lower annular hydraulic fluid storage chamber. As hydraulic fluid escapes from the cylinder into the lower annular storage chamber, the separator ring is forced upwards, thereby compressing the gas stored in the upper chamber. The compressed gas thus functions like the spring of a hydraulic accumulator which retains the balance of pressure throughout the system, and which provides a return force to move the piston to its upwards, retracted position, when tension on the piston rod is reduced by removing the load. This return force function is enabled by the novel design of the down-valve, as will now be described.
The down-valve is so constructed as to permit passage of hydraulic fluid from the lower annular storage chamber back into the inner cylinder when the load is removed. In other words, when the load is removed, the down valve, which previously was manually actuated to allow passage of fluid from the cylinder to the lower annular storage chamber, now functions automatically as a dump-valve to allow passage of fluid from the lower annular storage chamber back into the inner cylinder. Downward pressure exerted on the separator ring by compressed gas in the upper chamber pressurizes hydraulic fluid in the lower annular storage chamber. Re-admission of hydraulic fluid thus pressurized into the inner cylinder forces the piston back upwards to a retracted position.
The auxiliary hoist control apparatus includes an up-pump for raising the piston and an attached heavy load in small, precisely controlled increments, e.g., one one-thousandths of an inch for a 12-inch travel piston supporting a 5-ton weight. The up-pump functions by withdrawing hydraulic fluid from the lower fluid storage chamber and injecting the fluid into the lower part of the inner cylinder, below the piston.
Loads in excess of twenty tons are accurately positionable using the auxiliary hoist control disclosed in Merrill et al., U.S. Pat. No. 3,025,702. That hoist control includes a return force gauge consisting of a pressure gauge which has an input port and gas passageway which communicates with the annular compressible gas storage area above the separator ring. The return force gauge is calibrated in pounds force exerted downwardly by the compressed gas on the separator ring. In a typical application of the auxiliary hoist control, nitrogen gas is introduced into an inlet port of the upper annular gas storage area at a pressure sufficient to provide a return force which is a fraction of the weight of a load to be positioned using the apparatus. For example, if an auxiliary hoist control of the type described has a capacity of 10,000 pounds and is being used to position a 5,000-lb. load, the upper annular gas storage chamber may be pressurized with nitrogen gas to provide a downward return force on the separator ring of 100 pounds force, as indicated on the return force gauge. In effect, adjusting the initial pressure within the upper annular gas storage chamber to produce a particular return force simulates the effect of having an adjustable spring constant compression spring disposed longitudinally within the upper annular gas storage chamber, between the upper surface of the separator ring and the lower, inner surface of the upper cylinder head.
The auxiliary hoist control disclosed in the '702 patent also has a hydraulic fluid pressure gauge mounted on the body of the apparatus. The gauge has a relatively large face which is readable from a substantial distance, is coupled through a fluid passageway to the lower end of the inner hydraulic fluid cylinder below the piston, and is calibrated in pounds force exerted by a load suspended from the lower end of the piston rod. Thus, a 5,000-lb. load which is hung from the support eye at the lower end of the piston rod exerts a downward tension load on the rod which in turn is exerted on the piston and causes the piston to exert downward pressure on hydraulic fluid between the piston and lower cylinder head of the apparatus, producing a weight reading of 5,000 lbs. on the load weight gauge.
Rather than merely indicating to an operator the static weight of a load suspended from the auxiliary hoist control, the load weight gauge serves an additional important function. Thus, if a load is being lowered in a mating operation towards an object with which the load is to be mated, by operation of the down valve, for example, the reading on the load force gauge should remain constant. However, if the load is subjected to any sort of a resisting force in its downward travel, such as colliding with an underlying obstruction or sticking within the bore of a pipe, etc., the load gauge reading will decrease. This negative load deviation indication provides an immediate indication to an operator of a problem in the positioning of the load which requires corrective action.
Similarly, when a load is being raised, as in a de-mating operation, an increase in the load gauge reading provides an immediate indication to an operator of an overlying obstruction problem which must be corrected. Thus it can be appreciated that load deviation indication by the load force gauge is an important function of the auxiliary hoist control. However, the use of a hydraulic pressure gauge in the auxiliary hoist control of the '702 patent is problematic for the following reason. When the up-pump is operated to raise a load, the injection of hydraulic fluid by the pump into the lower portion of the inner cylinder below the piston causes a transient dynamic over-pressure in the hydraulic fluid. The magnitude of the transient dynamic over-pressure can be a significant fraction of total force reading on the load force gauge, e.g., 200 lbs. For a 5,000 lb. Load. The transient pressure increase indication gives a false load deviation indication to an operator, thus requiring him to interrupt what was a normal lifting operation to search for a non-existent problem. The present invention was conceived to provide a precision load positioner apparatus which incorporates desirable features of the auxiliary hoist control disclosed in the '702 patent, but which also provides an essentially error-free, immediate indication of load deviation and a mechanism to prevent over-pressuring the apparatus.